title: Mitigate B1+ inhomogeneity using spatially selective radiofrequency excitation with generalized spatial encoding magnetic fields.
abstract: OBJECTIVE
  High-field magnetic resonance imaging (MRI) has the challenge of inhomogeneous B(1)(+), and consequently inhomogeneous flip angle distribution, which causes spatially dependent contrast and makes clinical diagnosis difficult.
  
  
  METHODS
  We propose a two-step pulse design procedure in which (1) a combination of linear and nonlinear spatial encoding magnetic fields (SEMs) is used to remap the B(1)(+) map in order to reduce the dimensionality of the problem, (2) the locations, amplitudes, and phases of spoke pulses are estimated in one dimension. The advantage of this B(1)(+) remapping is that when the isointensity contours of a linear combination of SEMs are similar to the isointensity contours of B(1)(+), a simple pulse sequence design using time-varying SEMs can achieve a homogenous flip-angle distribution efficiently.
  
  
  RESULTS
  We demonstrate that spatially selective radiofrequency (RF) excitation with generalized SEMs (SAGS) using both linear and quadratic SEMs in a multi-spoke k-space trajectory can mitigate the B(1)(+) inhomogeneity at 7T efficiently. Numerical simulations based on experimental data suggest that, compared with other methods, SAGS provide a formulation allowing multiple-pulse design, a similar average flip-angle distribution with less RF power, and/or a more homogeneous flip-angle distribution.
  
  
  CONCLUSIONS
  Without using multiple RF coils for parallel transmission, SAGS can be used to mitigate the B(1)(+) inhomogeneity in high-field MRI experiments.
authors.affiliation: Department of Mathematics, National Taiwan University, Taipei, Taiwan; Department of Biomedical Engineering and Computational Science, Aalto University School of Science, Espoo, Finland.
authors.forename: Yi-Cheng
authors.initials: YC
authors.lastname: Hsu
authors.forename: I-Liang
authors.initials: IL
authors.lastname: Chern
authors.forename: Wei
authors.initials: W
authors.lastname: Zhao
authors.forename: Borjan
authors.initials: B
authors.lastname: Gagoski
authors.forename: Thomas
authors.initials: T
authors.lastname: Witzel
authors.forename: Fa-Hsuan
authors.initials: FH
authors.lastname: Lin
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001921:Brain / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D016247:Information Storage and Retrieval / Q000379:methods*; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: 7T; RF inhomogeneity; SAR; fast imaging; nonlinear gradient